It would be horribly overpowering if you could cast that in combat. But if you could only cast it outside combat. It would mostly save time. So let's also add group dscales, group blur, group vit and group realms :P

The conditions would be that you should displace the tanks first via target displace, then group displace the rest of group because group displace should not receive the illusion specialization :) I don't think it'd be overpowered if done that way.